Even i used to have severe stomach ache when i was young.. Tender coconut on an empty stomach will help.. I travelled to my village during summer holidays and had the chance of having tender coconut everyday morning.. and next time when i had periods i didnt have any pain.. since then i used to take tender coconuts almost everyday.. if it is not possible everyday, i used to take that compulsorily atleast 10 days ahead of the periods.. used to help a lot.. I also read in some siddha books that tender coconuts help a lot if taken continously on an empty stomach..
we need to keep the body cool.. Oil bath once in a week will balance the heat in the body and may help.. Fenugreek is also good.. Buttermilk with a little bit of hing will also work..
nutritious food is a must.. whenever i used to diet i used to have severe pain.. but when i have a complete meal.. i never used to have any discomfort..
Wheat kanji provides enough stamina during this periods and wards of back ache.. ask your sister to try that.. If your sister is having a lot of spicy food, ask her to reduce it.. or balance it with buttermilk etc.,
If the home remedies doesnt work for a while.. suggest that you check with doctor once.. if the doc says everything is normal and this is usual.. you can carry on with the home remedies itself..